Control systems for power devices



y 3, 1960 R. a. QUICK 2,935,661

CONTROL SYSTEMS FOR POWER oavxcss Filed May 2, 195'! P001 12 fine 1x17INVENTOR flan?! $07M 261 a 1? BY- Jmv M.

ATTORNEYS,

United States Patent CONTROL SYSTEMS FOR POWER DEVICES Robert G. Quick,Richmond, Va., assignor to Flight Research, Incorporated, a corporationof Virginia Application May 2, 1957, Serial No. 656,536

4 Claims. (Cl. 317-148.'5)

This invention relates to power controlling apparatus, and, moreparticularly, to apparatus including a meter movement relay forcontrolling the application of power.

One of the most sensitive devices for control purposes is a metermovement relay, preferably of the Well-known DArsonval type. This typeof relay, despite the advantage of its high sensitivity, also has verylow torque output, so that, when it is used in power applications, somemeans for protecting the contacts of the relay must be found.Particularly when the apparatus is subject to vibration, the resultantseparations of the contacts of the meter movement relay cause arcsbetween those contacts which eventually build up into carbon depositswhich finally prevent contact closure of the relay. Also, during theclosing movement of the relay contacts, the stray capacity of the switchelements and the associated wiring causes transient currents to flowthrough the switch contacts, which currents frequently result in thecontacts being welded together. Since the restorer spring must have alow torque, the welding results in the contacts remaining closed anderrors in indication result. Moreover, vibration of the mounting meansfor the relay causes chattering of the contacts so as to result in improper operation of the device being controlled by the relay.

The prior art has proposed solutions to these problems, one of which isto provide the stationary contacts of the meter movement relay withpermanent magnets and to provide the pointer, or movable contact, of therelay with an iron tip. When the contacts engage, magnetic attractionholds them engaged, to supplement the torque imposed on the pointer bythe current flowing through the coil of the meter movement. While thisproposal does result in more perfect engagement of the contacts, somemechanical device is necessary to interrupt the contacts, because thetorque caused by current through the metered coil will not ordinarily besufficient to separate the contacts against the permanent magnetattraction. This proposal, consequently, is not completely satisfactory,para supplemental winding on the relay which is normally I open butwhich is provided with current when the contacts close. The supplementalwinding provides an increased torque which tends to maintain thecontacts closed. While better engagement and protection againstchattering is provided by this proposal, the proposal necessitates anadditional circuit for interruption of the supplemental winding circuitwhen the meter movement contacts are to be opened. Further, the requiredcurrent through the contacts of the relay in order to control thesupplemental winding causes deterioration of the contacts and limitedlife for the relay.

While the apparatus of the present invention has broad utility in anycontrol system for a power device, it was especially designed to controlautomatically the aperture setting of a motion picture camera. For suchuse, a con Patented May 3, 1960 "ice trol had to be designed which wouldbe responsive to a current of no more than 2.5 micro amperes. For anillustration, 1,000 ohms coil of a DArsonval relay, a current of thismagnitude would provide approximately 6 thousandths of a microwatt ofpower. It is evident that the torque available to close the contacts ofthe DArsonval movement is extremely small, so that some type ofapparatus for maintaining the contacts closed after closure, or forcompensating for chattering, had to be found in order to provide properoperation of the motor which controlled the aperture setting of thecamera.

It was found that, if the contacts of the DArsonval relay were allowedto separate but their separations compensated for by use of a time delaycircuit, the necessity for such devices in the prior art as referred toabove, was avoided. It was also found that a current-limiting resistorin series with the contacts would prevent high transient currentsthrough the contacts and improve their useful life. However, the problemof how to control a motor, which is a relatively high power device ascom pared with the power available from the DArsonval movement, stillremained. The present invention utilizes a switching device, such as atransistor, between the DArsonval control system and the motor system,with the output of the switching device operable to control a relaywhich operates the power device.

It has'been found that the combination of elements referred to abovepermits satisfactory and positive operation of a power device for a longperiod of time without complicated or expensive circuitry and withrelatively small volume required for the elements of the controlapparatus.

The apparatus of the invention, generally speaking, in cludes a metermovement relay with at least a movable contact and a fixed contact, acontrol circuit for the relay, a power relay for controlling the powerdevice, a source of potential connected to the power relay when aswitching device is operated, and a circuit including an R-C time delaycircuit and the contacts of the meter movement connected to theswitching device to control its operation. As indicated above, acurrent-limiting resistor is also connected in series with the contactsof the meter movement relay, and the capacitor is connected across theseries combination of the contacts and this resistor.

The apparatus of the invention will now be more fully described inconjunction with a preferred embodiment thereof, as shown in theaccompanying drawing.

In the drawing,

Fig. l is a schematic diagram of the complete control system of apreferred embodiment of the invention; and

Fig. 2 is a simplified schematic of the apparatus of Fig. 1 showing aportion thereof, so as to permit a more ready explanation of theoperation of the apparatus.

Referring first to Fig. 1, the power controlling apparatus is itselfcontrolled by a control circuit 1 which provides current from a sourceof potential indicated as battery 2, to a meter movement relay 3, whichis preferably of the DArsonval type. The meter movement relay includes acoil 4 connected to the output of con trol circuit 1, and a movablecontact 5 which selectively engages stationary contacts 6 and 7. Themeter movement relay is intended to control a pair of power relays 8 and9 whose contact sets are connected toa power circuit 10 containing thedevice to be controlled. For instance, the power circuit 10 may includea motor for performing any appropriate function, such as controlling theaperture setting of a motion picture camera. As an illustration ofpractical use of the power relays to control a power device, the relaysmay be connected to be inoperable to drive the motor when both are.unenergized, and one relay when energized-may closea circuit to drivethe motor in one direction and the other relay when energized may closeanother circuit to drive the motor in the reverse direction.

Power relays 8 and 9 are in the output circuits of transistors 11 and12, one side of the coils of power relays 8 and 9 being connected to thecollector elements of the respective transistors, and the other sides ofthe coils being connected to the negative side of battery 2 by conductor13. The emitters of the transistors 11 and 12 are connected by lead 14to the positive side of battery 2. Therefore, when sufiicient current isflowing through either of the emitter-collector circuits of thetransistors, the associated power relay is energized.

The input circuit of transistor 11, the control circuit therefor,includes movable contact and stationary contact 6 of the meter movementrelay 3. The stationary Contact is connected through a resistor 15 tolead 16 which is connected to the base of transistor ll. Lead in isconnected through the series combination of a stabilizing resistor 17and a thermistor 18 (whose purpose will subsequently be explained) tothe emitter of the transistor.

Similarly, stationary contact 7, in the input circuit of transistor 12,is connected through resistor 19 and lead 20 to the base of thetransistor, while lead 20 is connected through stabilizing resistor 21and thermistor 22 to the emitter of transistor 12.

A pair of capacitors 23 and 24 are connected across stationary contacts6 and '7, with their common lead being connected to lead 13, and henceto the negative side of battery 2.

A current-limiting resistor 25 is connected between the movable contact5 of the meter movement relay and lead 13. This resistor is preferablyplaced immediately adjacent the contacts so as to prevent the straycapacitance between them from furnishing an unlimited surge currentthrough them on closure.

In order that the operation of the apparatus of Fig. 1 may be moreeasily understood, reference is now made to Fig. 2 which is a schematicof substantially one-half of the control system of Fig. 1. That is, Fig.2 shows the circuit of transistor 11 and its power relay 8, togetherwith the meter movement relay contacts and the rest of the controlcircuit for that transistor. When the contact 5 is out of engagementwith contact 6 of the apparatus of Fig. 2, the connections and thetransistor selected are such that there is no appreciable current how inthe emitter-collector circuit of the transistor, so that power relay 8is not energized.

An important feature of the invention is that the entire potential ofbattery 2, plate potential, is connected across the meter relay contactsbefore they engage. This is necessary in order that the resistance ofoxide and dust films may be broken down, despite the relatively lowtorque supplied the pointer of the relay by the low control current, inorder to provide a low resistance circuit in the input of the transistorswitching device.

When movable cont-act 5 moves over to engage contact 5, a circuit to thenegative side of battery 2 is completed, and the transistor is cut-on,to furnish sufiicient current through the power relay to energize it.The time interval after closure of the switch contacts before thetransistor is cut-on depends upon the magnitude of the capacitor 23 andresistor 25, which latter functions to discharge capacitor 23. Duringtheclosure operation and subsequent operation of the apparatus, resistor25 limits the current which can fiow through contacts 5 and 6 to a valuesuch that the life of the contacts is increased as far as possible.

When vibration or decrease of current through the coil 4 of the metermovement relay causes contacts 5 and 6 to separate, capacitor 2.3 andresistor 15 form an R4? time delay circuit which keeps the transistorcut-on for a time period after opening of the contacts determined by thevalues of the respective elements. The values are preferably chosen sothat the time constant of the circuit is substantially longer than thelongest period of vibration to be expected with the apparatus, so thatcontacts 5 and 6 will close again before the R-C circuit 23 and 15permits the transistor 11 to cut-olf and de-energize relay 8.'

Thermistor 18 is provided to insure that the quiescent current of thetransistor will be negligible, that is, that the current through theemitter-collector circuit when the switch contacts are open in staticcondition will be nil.

It will be evident from the above explanation of the operation of Fig. 2how the apparatus of Fig. l operates. When contacts 5 and 6 close,transistor 11 is cuton to energize power relay 8, while, when contacts 5and 7 close, transistor 12 is cut-on to energize power relay 9. Theappropriate connections are made in power circuit 19 to influence thepower device in any desired manner. As with capacitor 23 and resistor15, capacitor 2-l and resistor 19 form an R-C time delay circuit fortransistor 12 to keep it energized for a selected time period afteropening of contacts 5 and 7. Thermistor 22 and resistor 21 are connectedin the circuit and to the battery in such fashion as to maintain thetransistor 12 cut-oi? until the contacts 5 and 7 close.

It will be noted that the R-C time delay circuit operative to keep eachtransistor on for a time period after opening of the associated'switchcontacts is in the input circuit of the transistor. Since the lowresistance in the output circuit would require a higher value ofcapacity for the same time constant, this placing of the time delaycircuit is much superior to having it in the output circuit.

In practical operation of the apparatus of the invention to control theaperture setting of a camera, for instance, the apparatus disclosed mayfunction for several months to provide continuous control under theinfluence of, for example, of a photocell, with only the power suppliedfrom simple dry batteries. The power drain from such a source to thecontrol apparatus would be minimum, because the transistors wouldconsume power only when they were turned on to elfect their controlfunction.

It will be evident that a different type of switching device than thetransistors shown could be employed. For instance, a vacuum tube couldbe employed and the circuit redesigned to cause it to operate in similarfashion to the transistor. However, particularly for applications wheresize and weight of components are important, as well as powerconsiderations, since filament power drain is not involved, thetransistor will be found to be immensely superior to the vacuum tube.Especially for a system for control of the aperture setting of a motionpicture camera, and even more importantly where such apparatus isdesigned to be carried by hand with the camera, the transistor is theonly practicable switching device presently known for this purpose.Either a transistor or a vacuum tube would be superior to any other typeof switching device however, because the amplification they providepermits low current through the contacts.

It will be evident that many different types of power devices could becontrolled by an apparatus of the type described herein. It will also beevident that many minor changes could be made in the circuitry of theapparatus without departure from the scope of the invention. Theinvention is therefore not to be considered to be considered to belimited to the embodiment disclosed in the application, but rather onlyby the scope of the appended claims.

I claim:

1. Apparatus for controlling a power device comprising a meter movementrelay having a movable contact and at least one fixed contact, a controlcircuit for the meter movement relay operable to supply sul'licientcurrent through the coil of the meter movement to .close the fixed andmovable contacts, a transistor, a source of D.-C. potential, a powerrelay, the coil of the power relay being connected between a first oneof the transistor elements and one side of the source of potential, asecond of the transistor elements being connected to the other side ofthe source of potential, a current-limiting resistor and a secondresistor, the series combination of the current-limiting and secondresistors and said contacts being connected between the third transistorelement and said Y one side of the source of potential, means includinga resistance means connecting the second and third transsistor elementstogether, said transistor being normally cut-ofi, and a capacitorconnected across the series combination of said contacts and saidcurrent-limiting resistor operable with said second resistor to form anRC time delay circuit, said contacts when closed being operable tocut-on said transistor to cause it to conduct sufiicient current betweensaid first and second elements to energize said power relay, said powerrelay being operable to control said power device, said R-C time delaycircuit being operable to keep said transistor cuton for a time periodafter said contacts open.

2. Apparatus as defined in claim 1 which said connecting means includesa thermistor.

3. Apparatus for controlling a power device comprising a meter movementrelay having a movable contact and a pair of stationary contactsalternately engageable by the movable contact, a control circuit for themeter movement relay operable to supply sufficient current through thecoil of the meter movement to cause the fixed contact to engageselectively one and the other of the fixed contacts, a pair of powerrelays, a source of D.-C. potential, a pair of transistors, each of thepower relays being connected between one of the elements of a differentone of the transistors and one side of the source of potential, a secondelement of each transistor being connected to the other side of thesource of potential, a current-limiting resistor connected between saidmovable contact and said one side of the source of potential, a pair ofcapacitors and a pair of resistors, one side of each of the capacitorsbeing connected to a different one of the stationary contacts and theother sides of the capacitors being connected to said one side of thesource of potential, one of said pair of resistors being connectedbetween a third element of one of the transistors and one of thestationary contacts and the other resistor being connected between athird element of the others transistor and the other stationary contact,and means including a resistance means connecting the third and secondelements of each transistor together operable to maintain thetransistors cut-off, the movable contact and said one stationary contactbeing operable when engaged to turn said one transistor on and operatesaid one power relay and the movable contact and said other stationarycontact being operable when engaged to turn said other transistor on andoperate said other power relay, said pair of capacitors and said pair ofresistors forming R-C time delay circuits operable to keep theirassociated transistors on for a period of time after the associatedcontacts open, said power relays being operable to control said powerdevice.

4. Apparatus as defined in claim 3 in which said connecting meanscomprises a pair of thermistors, one in each transistor circuit.

Keeler Feb. 10, 1942 Gilbert Nov. 20, 1951

